This invention is directed to apparatus for the measurement of conductivity of a liquid and in particular, to a salinometer for the measurement of the ratio of the conductivity of saltwater to that of standard seawater.
In U.S. Pat. No. 3,963,979, which issued on June 15, 1976, to T. M. Dauphinee, a laboratory salinometer is described. It is based on continuous flow of sample water at a constant temperature, ensured by a heat exchanger and thermostatted bath, through the measuring cell, rather than measurements on discrete samples as was the case with prior systems. The advantages of the system were that complicated temperature compensations in the measuring circuit were eliminated while the continuous flow greatly simplified flushing of the system to remove all traces of the previous sample. This instrument in its commercial version, the Guildline Autosal, has now become the standard instrument in many oceanographic laboratories around the world.
A sophisticated version of the same system, using two heat exchanger-cell units in two different thermostatted baths, was developed for studies of the conductivity of seawater as a function of salinity and temperature, and of the KCl equivalent of standard seawater. The results obtained led to the form of equations used for the Practical Salinity Scale, 1978, which is described in the publication by E. L. Lewis, "The Practical Salinity Scale 1978 and its Antecedents", IEEE Journal of Oceanic Engineering, Vol. OE-5, pp 3-8, January 1980, and make up a substantial part of the data on which the new scale is based.
The bulk and weight of the Autosal, which resulted largely from the thermosetting system, made it unsuitable for use in any but a fixed location or the laboratory of a relatively large ship. However, a need is apparent for an instrument that is suitable for field and small boat use, can be easily transported from place to place, requires much less sample than the 100-200 ml needed by the Autosal and other salinometers, but is still accurate enough to satisfy most oceanographers' needs.